very basic doorbell

On a sunny day (Mon, 24 Sep 2007 18:41:43 -0000) it happened snipped-for-privacy@gmail.com wrote in :

That likely means it uses 10mA when it is connected to 12 V. You do not need and do not want the resistor. The thing itself is about 1200Ohm

and plan on

A battery with enough Ah and a low enough Ri together with a short in the buzzer *could* do it. If you are paranoid use a 50mA fuse in series.

thanks for the response. I just realized I should have posted in sci.electronics.basics - sorry about that.

What you calculated is the total resistance; that is the value of the external resistor, plus the resistance of the buzzer As somebody poined out, the buzzer is probably about 1200 Ohms by itself. That would be how they got the 10 ma.

Tam

cool - got it. One more question, how is a 50mA fuse determined? That is, why a 50mA, not 40, etc.

thanks a lot...

The fuse value is not particularly critical - you want it large enough that the fuse will not blow during normal operation, but small enough that a true fault will blow it. You also want a commercially-available value, which may be why 50 mA was suggested, rather than 40 mA.